Traditional sensors for measuring a physical phenomenon, like a blood oxygenation, are usually designed for direct use on a single body-part. For example the sensors are designed as finger-clips, ear-clips, forehead-sensors, toe-clips or ankle-sensors comprising infrared light sources and infrared detectors. These sensors are shaped such that they fit perfectly on the according body-part. Each body part has different light reflection and/or light transmission characteristics due to the inner structure of the skin, for example the skin is thicker on feet than on face leading to higher amplitude of measurements of the physical phenomenon on the face then on the feet. To overcome these differences, predetermined calibration curves are used, wherein the calibration curves are adapted for the body-part on which the measurement is taken. The correct calibration curve can be selected easily for the sensors described above, since the sensors are clearly dedicated to a specified body part. After selecting the correct calibration curve, the measured values are calculated from a raw signal obtained by the traditional sensor using the selected calibration curve.
In camera-based vital signs monitoring physical phenomena can be measured unobtrusively from a distance. A problem is that in camera-based vital signs monitoring it cannot be concluded from different sensor designs which body part is actually measured. Hence, specific calibration curves cannot be selected directly. However, a body part independent adjustment does not yield the accuracy required by healthcare professionals.